An atomic force microscope (AFM), also referred to as a scanning force microscope (SFM), is an instrument in which a flexible cantilever of very small dimensions is scanned relative to a surface. Such cantilevers typically have a pointed tip which projects from the free end of the cantilever in the direction of the sample surface. As the sample is scanned, forces between the sample surface and the cantilever tip cause the cantilever to deflect, and the topography of the surface is measured by monitoring the deflection of the cantilever. The forces between the sample and the cantilever tip include electrostatic, magnetic, viscous, van der Waals (both repulsive and attractive) and other forces. The cantilever may be used in a contacting mode of operation, in which the tip is in contact with the sample surface, or a non-contacting mode, in which the tip is maintained at a short distance, typically from 5 to 500 .ANG. or more, from the surface of the sample.
AFMs are a single variety of a much broader class of instruments referred to as "scanning probe microscopes" (SPMs). As used herein, "scanning probe microscope" means an instrument which provides a microscopic analysis of the topographical features or other characteristics of a surface by causing a probe to scan the surface. It refers to a class of instruments which employ a technique of mapping the spatial distribution of a surface property, by localizing the influence of the property to a small probe. The probe moves relative to the sample and measures the change in the property or follows constant contours of the property. Depending on the type of SPM, the probe either contacts or rides slightly (up to several 100 .ANG.) above the surface to be analyzed. In addition to AFMs, scanning probe microscopes include devices such as scanning tunneling microscopes (STMs), scanning acoustic microscopes, scanning capacitance microscopes, magnetic force microscopes, scanning thermal microscopes, scanning optical microscopes, scanning ion-conductive microscopes and others.
While the exact design of the cantilever depends on its application, in general the following properties are desirable. The cantilever should have a mass as low as possible for a given length and area and exert an extremely light tracking force on the sample, on the order of 10.sup.-5 to 10.sup.-10 N, in order to avoid damage to the sample. The force constant of the cantilever should be extremely low, on the order of 0.001 N/m to 250 N/m. The cantilever should have a high mechanical resonance frequency, preferably above 10 kHz, to permit fast sample scanning rates. When the AFM is used in non-contact modes, a well defined resonance frequency and low damping (high mechanical Q) are also desirable, to maintain high sensitivity to changes in the resonant frequency of the cantilever. Ideally, a very small point or portion at the apex of the tip interacts with the sample, so that the forces between the cantilever and sample are localized. The cantilever should also be free and exposed, for unobstructed access to the sample.
The cantilever tip should have small dimensions as compared with the features of the sample being analyzed. In this regard, the "aspect ratio" is defined as the ratio of the length of the tip to the width of its base, and for a tip having nonlinear walls the "average aspect ratio" is defined for a distance from the apex of the tip that is considered significant (e.g., about 1-2 .mu.m). The "tip radius" is defined as the radius of curvature of the tip at its apex. In most applications, the aspect ratio should be as high as possible and the tip radius should be as small as possible, so that a minimal portion of the tip interacts with the sample surface and senses the interaction force. For AFMs which are to be used also as scanning tunneling microscopes (STMs), the cantilever and tip must be electrically conductive, so that a tunneling current can flow between the tip and the sample.
Cantilever tips can be produced by a variety of techniques, for example, by gluing tip fragments to the free end of the cantilever, or by depositing material for the tip. Microfabricated cantilevers with integral tips can also be produced. U.S. Pat. No. 4,968,585 describes a method of microfabricating SiO.sub.2 cantilevers with integrated conical silicon tips. U.S. Pat. No. 5,021,364 describes a method of microfabricating Si.sub.3 N.sub.4 cantilevers with integral tetrahedral silicon tips. Both of the foregoing patents indicate that it is desirable to have a tip with a radius of less than 500 .ANG.. Generally speaking, the integral tips in most prior art cantilevers have relatively low aspect ratios and large tip radii, as compared with the sample features being imaged.
Several techniques have been used to detect the deflection of the cantilever as the sample surface is scanned. These techniques include interferometry, optical beam deflection, capacitive sensing, electron tunneling, and piezoresistive detection. Most of these deflection detection systems are external to the cantilever, and they are bulky and require time-consuming fine alignments. An exception is the piezoresistive system, which uses a piezoresistor which is attached to or embedded in the cantilever. Piezoresistive systems derive a signal representative of the bending of the cantilever directly from the cantilever itself, with no external deflection measuring components, and they require many fewer adjustments and alignments than the external systems.
The cantilever of this invention overcomes many of the problems and disadvantages of prior art cantilevers and detection systems.